Recently, much attention has been focused on GaN-based compound semiconductors (e.g., GaxAl1xe2x88x92xN or InxGa1xe2x88x92xN, where 0xe2x89xa6xxe2x89xa61) for blue and green light emitting diode (LED) applications. One important reason is that GaN-based LEDs have been found to exhibit excellent light emission at room temperature.
In general, GaN-based LEDs comprise a multilayer structure in which n-type and p-type GaN are stacked on a substrate (most commonly a sapphire substrate), and InxGa1xe2x88x92xN/GaN multiple quantum wells are sandwiched between the p-type and n-type GaN layers. A number of methods for growing the multilayer structure are known in the art, including metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
It is also known in the art that these conventional growth methods for compound semiconductor structures have proven problematic, particularly with respect to forming a p-type GaN-based layer suitable for LED applications. In general, GaN layers formed by known growth methods, such as MOCVD, and doped with p-type material such as magnesium, behave like a semi-insulating or high-resistive material. It is generally understood that this results from hydrogen passivation, or hydrogen that is present in the reaction chamber bonding with the p-type dopant and thus preventing the dopant in the GaN from behaving as an active carrier. Because of this phenomenon, p-type GaN having a sufficiently low resistivity to form the p-n junction required for LED applications cannot be produced by conventional techniques.
Various attempts have been made to overcome the difficulties in obtaining p-type GaN-based compound semiconductors. In one technique known as low-energy electron-beam irradiation (LEEBI), a high-resistive semi-insulating GaN layer, which is doped with a p-type impurity (Mg), is formed on top of the multilayers of the GaN compound semiconductor. Then, while maintaining the semiconductor compound at temperatures up to 600xc2x0 C., the compound is irradiated with an electron beam having an acceleration voltage of 5-15 kV in order to reduce the resistance of the p-doped region near the sample surface. However, with this method, reduction in the resistance of the p-doped layer can be achieved only up to the point that the electron beam penetrates the sample, i.e. a very thin surface portion less than about 0.5 xcexcm deep. Furthermore, this method requires heating the substrate to temperatures up to approximately 600xc2x0 C. in addition to high-voltage acceleration of the electron beam.
Thermal annealing can be used to activate a small fraction of the dopant as an active carrier. For example, in order to achieve a carrier concentration of 1xc3x971018 cmxe2x88x923, the concentration of the p-type dopant must be as high as 1xc3x971020 cmxe2x88x923.
Also, the high level of doping required for the thermal annealing method degrades Hall mobility of the p-type GaN, with a typical value of Mg-doped GaN of only 20 cm2/v-s. Moreover, because of the heavy doping and the degraded top layer crystallinity, the forward bias voltage of GaN-based LEDs cannot be made as low as desired, and the light-emitting efficiency is decreased.
Furthermore, the annealing temperature is typically more than 800xc2x0 C., which is higher than the temperature used for forming the light emitting layers. These high temperatures may additionally degrade the light-emitting efficiency of the device.
A method for activating a dopant in a semiconductor comprises irradiating the semiconductor with x-ray radiation. This process can be used to fabricate low-resistance p-type compound semiconductors for example, including III-V Group compound semiconductors, such as GaN-based semiconductors. The x-ray radiation may be generated by an x-ray source and directed to the surface of the doped compound semiconductor wafer. In certain embodiments, the x-ray source has an accelerating voltage of approximately 40 kV and an intensity of approximately 0.4 mA, for example. Additionally, the position of the sample may be changed with respect to the incident x-ray radiation so that the incident angle of the x-ray radiation sweeps through a limited angular range during the irradiation process. According to at least one embodiment, the limited angular range is approximately 100-5000 arc sec.
In contrast to known methods for activating dopants in semiconductor materials, this method can be performed at room temperature in an atmospheric, clean room, or processing chamber environment. The time duration of the x-ray irradiation may also be selectively controlled to maximize the carrier concentration of the irradiated samples. Typically, when using an x-ray system such as Model QC1A available from Bede Scientific, Inc., an irradiation time between approximately 4 and 7 minutes is used to achieve the desired dosage.
The present invention further relates to a method for improving the doping efficiency of p-type compound semiconductors. According to certain embodiments, a compound semiconductor is doped with a p-type impurity, such as magnesium, and having a dopant concentration of approximately 1xc3x971019 cmxe2x88x923, is irradiated with x-ray radiation to produce a p-type compound semiconductor with a carrier concentration of 1xc3x971018 cmxe2x88x923.
A method of fabricating a p-type compound semiconductor is additionally disclosed, the method comprising the steps of growing a compound semiconductor by using reaction gas containing p-type impurities, and then irradiating the compound semiconductor with x-ray radiation to activate the p-type impurity.
A method of fabricating a compound semiconductor light emitting diode (LED) is further disclosed, the method comprising the steps of growing a compound semiconductor LED structure using a known epitaxial growth process, and then irradiating the LED with x-ray radiation to activate a p-type impurity. The LED may be a GaN-based LED, and in particular embodiments, a GaN-based blue LED. In general, the LEDs produced with the x-ray irradiation method of the present invention demonstrate improved forward bias voltage and light-emitting efficiency over previously known methods.